1. Field of the Invention
The present invention relates generally to optical head devices for use in recording and/or reproduction of information, and more particularly, to an optical head device which causes a light beam to impinge upon a record medium and guides the light beam from the record medium to a photodetector for reproducing information recorded on the record medium and obtaining a tracking error signal used for maintaining the light beam incident upon the record medium in correct tracking relation to a record track formed in the record medium.
2. Description of the Prior Art
In an optical disc player for reproducing information recorded on an optical record disc, an optical head device is provided within an optical arrangement to read an information signal from a record track formed in he optical record disc by the use of a laser light beam impinging upon the record track. The optical head device is required to produce the laser light beam, to cause the laser light beam to be focused correctly on the record track, which is usually very narrow in width, so as to trace faithfully the same, and to guide properly a reflected laser light beam obtained from the optical record disc to a photodetector. For fulfilling these requirements, the optical head device comprises a precise arrangement of various optical components including a semiconductor laser for generating a laser light beam, an objective lens facing the optical record disc to focus the laser light beam incident upon the optical record disc and to receive the reflected laser light beam from the optical record disc, other lenses, mirrors, prisms, photosensitive elements constituting the photodetector and so on. These components are supported to be movable in both directions along an optical axis of the objective lens and perpendicular to the optical axis of the objective lens in order to maintain focus control so that the laser light beam incident upon the optical record disc remains in correct focus relation to the record track and to maintain tracking control, so that the laser light beam incident upon the optical record disc remains in correct tracking relation to the record track.
In the focus control performed in the optical head device, a focus error signal which varies in response to variations in the focus condition of the laser light beam incident upon the record track in the optical record disc is produced based on detection output signals obtained from the photodetector to which the reflected laser light beam from the optical disc is guided and the objective lens is moved along its optical axis in accordance with the focus error signal. Further, in the tracking control performed in the optical head device, a tracking error signal which varies in response to variations in the tracking condition of the laser light beam incident upon the record track in the optical record disc is produced based on the detection output signals obtained from the photodetector to which the reflected laser light beam from the optical disc is guided. The optical axis of the objective lens is centered onto the record track in accordance with the tracking error signal.
For producing the focusing and tracking error signals, the photodetector provided in the optical head device comprises, for example, four photosensitive elements d.sub.1, d.sub.2, d.sub.3 and d.sub.4 disposed close to one another, as illustrated in FIG. 1. The photosensitive elements d.sub.1 to d.sub.4 are so arranged that the photosensitive elements d.sub.1 and d.sub.3 are aligned along a direction X corresponding to the tangential direction of the record track in the optical record disc and the photosensitive elements d.sub.2 and d.sub.4 are also aligned along the direction X. The reflected laser light beam coming through the objective lens from the record track in the optical record disc forms a beam spot Q on the photosensitive elements d.sub.1 to d.sub.4 as indicated by a dot-dash line in FIG. 1. The photosensitive elements d.sub.1 to d.sub.4 produce respective detection output signals S.sub.1, S.sub.2, S.sub.3 and S.sub.4 each dependent on a portion of the beam spot Q formed on each of the photosensitive elements d.sub.1 to d.sub.4.
A reproduced information signal is produced by summing up the detection output signals S.sub.1 to S.sub.4, and the focus error signal is obtained based on a difference between an added signal obtained by adding the detection output signal S.sub.1 to the detection output signal S.sub.4 and an added signal obtained by adding the detection output signal S.sub.2 to the detection output signal S.sub.3, in the same manner as disclosed in, for example, U.S. Pat. No. 4,023,033 and U.S. Pat. No. 4,079,247.
The tracking error signal will now be considered. The record track in the optical record disc is composed of a series of pits having a depth which is a quarter of the wavelength .lambda. of the laser light beam incident upon the record track. The laser light beam irradiating the record track is diffracted and reflected by the pits. Then, the laser light beam which is modulated by the record track is reflected back through the objective lens as the reflected laser light beam and reaches the photodetector to form the beam spot Q on the photosensitive elements d.sub.1 to d.sub.4. Accordingly, the reflected light beam produces a diffraction pattern corresponding to the positional relationship between one of the pits forming the record track and a beam spot formed by the laser light beam irradiating the record track. The diffraction pattern thus produced is moved in a direction corresponding to the direction X in relation to the photosensitive elements d.sub.1 to d.sub.4 at speed corresponding to the movement of the record track relative to the laser light beam irradiating the record track.
FIGS. 2A, 2B and 2C illustrate the manner in which the foregoing process takes place. In each of FIGS. 2A to 2C, a designates positional relationships between a pit P and a beam spot L formed by the laser light beam irradiating the pit P, and b designates diffraction patterns (hatched portions) formed on an exit pupil surface of the objective lens by the reflected laser light beam in dependence on each of the positional relationships designated by a. Four quarters d.sub.1, d.sub.2, d.sub.3 and d.sub.4 shown at b indicate portions in which quantities of light are detected by the four photosensitive elements d.sub.1, d.sub.2, d.sub.3 and d.sub.4, respectively. The pit P moves from a position t.sub.1 to a position t.sub.2 in relation to the beam spot L. FIG. 2A shows a condition in which the beam spot L is displaced to the right in relation to the pit P. FIG. 2B illustrates a condition in which the beam spot L is positioned centrally on the pit P, namely, a correct tracking condition in which the laser light beam arrives properly at the record track in the optical record disc. FIG. 2C shows a condition in which the beam spot L is displaced to the left in relation to the pit P.
It is understood from FIGS. 2A to 2C that when the beam spot L is centrally positioned on the pit P, that is, when the laser light beam incident upon the record track is in the proper tracking condition, a diffraction pattern in which quantities of light are symmetrically distributed among another quarters d.sub.1 and d.sub.2 and among another quarters d.sub.3 and d.sub.4, is produced, and when the beam spot L is displaced rightward or leftward in relation to the pit P, a diffraction pattern is produced in which the distribution of the quantities of light among the quarters d.sub.1 and d.sub.2 and among the quarters d.sub.3 and d.sub.4, is asymmetric, with the light distribution out of symmetry in reversed patterns in the case where the beam pattern is disposed to the right and in the case where the beam pattern is disposed to the left, respectively. Based on the foregoing, the detection output signals S.sub.1 to S.sub.4 obtained from the photosensitive elements d.sub.1 to d.sub.4, respectively, may be supplied to and processed by a signal generating circuit for producing a signal which varies in dependence on the positional relationships between the beam spot L and the pit P, namely, the tracking error signal.
FIG. 3 shows one example of the signal generating circuit by which the tracking error signal is produced based on the detection output signals S.sub.1 to S.sub.4 obtained from the photosensitive elements d.sub.1 to d.sub.4, together with the reproduced output of an information signal recorded on the record medium, that is, a reproduced information signal.
In the circuit shown in FIG. 3, the detection output signals S.sub.1 and S.sub.4 obtained from the photosensitive elements d.sub.1 and d.sub.4, respectively are added to each other by an adder 11, and the detection output signals S.sub.2 and S.sub.3 obtained from the photosensitive elements d.sub.2 and d.sub.3, respectively, are added to each other by an adder 12. Output signals from the adders 11 and 12 are supplied to an adder 13 to produce an output signal S.sub.5 and supplied also to a subtracter 14 to produce an output signal S.sub.6. The output signal S.sub.5 obtained from the adder 13 serves as the reproduced information signal, and the output signal S.sub.6 obtained from the subtracter 14 comprises a signal residing in a frequency band of the information signal recorded on the record track in the optical record disc and varying whenever the beam spot L on the optical record disc passes through the pit P and serves as the tracking error signal which indicates deviations of the beam spot L from the center of the record track formed by the series of pits.
Further, in the circuit shown in FIG. 3, the output signal S.sub.5 obtained from the adder 13 is supplied to pulse generators 15 and 16. The pulse generator 15 produces a pulse signal S.sub.7 in response to each rising edge of the output signal S.sub.5 and the pulse generator 16 produces a pulse signal S.sub.8 in response to each falling edge of the output signal S.sub.5. The output signal S.sub.6 obtained from the subtracter 14 is supplied to sampling-hold circuits 17 and 18. In the sampling-hold circuit 17, the level of the output signal S.sub.6 is sampled by the pulse signal S.sub.8 and each level sampled by pulse signal S.sub.8 is held to produce an output signal S.sub.9. Similarly, in the sampling-hold circuit 18, the level of the output signal S.sub.6 is sampled by the pulse signal S.sub.7 and each level sampled by pulse signal S.sub.7 is held to produce an output signal S.sub.10. Each of the output signals S.sub.9 and S.sub.10 obtained from the sampling-hold circuits 17 and 18, respectively, has a polarity varying from negative to positive or vice versa when the beam spot L is moved to traverse the record track and a level representative of the deviations of the beam spot L from the center of the record track. Consequently, the output signals S.sub.9 and S.sub.10 can be employed as tracking control signals. These output signals S.sub.9 and S.sub.10 are supplied to a differential circuit 19 to produce a tracking control signal S.sub.11 which is more reliable and is then delivered to an output terminal 20.
In the previously proposed optical head device as described above, wherein the tracking error signal is produced in such a manner as aforementioned, four photosensitive elements constituting the photodetector are required to be precisely arranged with respect to each other and with respect to the record track in the optical record disc. This, results in complicated configurations of the photodetector itself and of devices for positioning the photodetector. Further, a bidirectional optical arrangement is required to control movement of the reflected laser light beam in each of two directions perpendicular to each other and to thereby accurately position the reflected beam onto the photodetector.
Especially, in the case where the previously proposed optical head device is equipped with an integrated light beam generating and detecting unit which comprises a semiconductor substrate on which a photodetector, a semiconductor laser, and a prism for directing a laser light beam generated by the semiconductor laser to impinge upon an optical record disc and for guiding a reflected laser light beam obtained from the optical record disc to the photodetector are provided, as disclosed in the Japanese patent application published before examination under publication number 62-197931, the disadvantage resulting from the photodetector composed of four photosensitive elements which are provided for producing the tracking error signal as described above is more serious.